A quickie refers to a brief or spontaneous episode of sexual activity, with the couple finishing in a very short amount of time.
Generally a couple will skip foreplay. A quickie does not require full copulation; it may be limited to oral sex or mutual masturbation. Quickies can be "a major turn-on" but still may not give a woman enough time to lubricate naturally.
Some consider quickies a solution to unequal sexual desire in a relationship, but if they become the only form of sex, the relationship may suffer.
For gay men, a quickie usually refers more specifically to a casual sex encounter such as a hand job, often taking the form of street cruising. Neither practice is especially common among lesbians, however.